After seeing that my wireless speeds were much faster than the speeds I was getting over Ethernet, I decided to invest in some new cables. I didn’t know it before, but I saw while I was changing them out that my current cables were Cat 5e. While putting my network together, I had just been grabbing whatever cables I could find in my scrap drawers. Now I have Cat 8 cables and my speeds jumped from 7MB/s to an average of over 40MB/s. It’s a much bigger improvement than I expected, especially for such a small investment.
True. Hence my caveat of “most cards”. If it’s got LEDs on the port, it’s quite likely to signal which speed it is at with those LEDs.
I haven’t yet come across a gigabit card that won’t do 10Mbit (edit: switches are a different matter) but sometimes I’ve come across cards that fail to negotiate speeds correctly, eg trying for gigabit when they only actually have a 4 wire connection that can support 100Mbit. Forcing the card to the “correct” speed makes them work.